gh-115304: Add doc for initialize PyMutex as global variables

[aisk]

• Issue: The typical initialization of PyMutex in its comment document cannot be compiled on Windows #115304

[colesbury]

I don't think we should make this change. The comment is for variables that need explicit initialization, such as local variables and fields in heap allocated structures. Static variables are implicitly initialized to zero in C.

[colesbury]

Maybe instead we can add an additional example of statically initializing the PyMutex?

[aisk]

Thanks for the review! For local variables, should we remove the cast (PyMutex)? Seems it is needless in modern C.

[colesbury]

Most commonly, we are initializing fields in a struct like:

self->mutex= (PyMutex){0};

You can't write:

self->mutex= {0}; // INVALID!

[aisk]

Ah, I've got the point!

[colesbury]

Thanks!